The invention relates to a colour display tube comprising in an evacuated envelope an electron gun system of the "in-line" type to generate three electron beams situated with their axes in one plane. The electron beams converge on a display screen provided on a wall of the envelope and, in the operating condition of the display tube, are deflected over the display screen in two mutually perpendicular directions by means of a first and a second deflection field. The direction of the first deflection field is parallel to the plane. The electron gun system comprises at its end field shapers to cause the frames described on the display screen by the electron beams to coincide as much as possible. The field shapers comprise substantially annular elements of a material having a high magnetic permeability and positioned around the two outermost beams.
Such a colour display tube is disclosed in U.S. Pat. specification No. 4,196,370. A frequently occurring problem in colour display tubes having an electron gun system of the "in-line" type is line coma and field coma, which causes the dimensions of the frames described on the display screen by the three electron beams to be different. This is the result of the eccentric location of the outermost electron beams with respect to the field for the horizontal and vertical deflections, respectively. In the above-mentioned Patent Specification a large number of Patents are mentioned in which partial solutions are given. These solutions consist of using rings and plates for conducting and/or screening magnetic fields which are mounted at the gun end and which intensify or weaken the deflection field or the deflection fields locally along a part of the paths of the electron beams.
For the deflection of the electron beams in colour display tubes various types of coils are used. In tubes having an "in-line" electron gun system, the coils usually are self-converging. One of the much used types of coils is the so-called hybrid coil. Thus consists of a saddle-shaped line coil and a toroidal field coil. As a result of the winding method used for the manufacture of the field coil it is not possible to make the coil so as to be entirely self-converging. A winding distribution is usually chosen which is such that a given convergence error remains, the so-called coma. This coma error is evidenced, for example, by a larger frame (horizontal and vertical) for the outermost beams than for the central beam. The horizontal and vertical deflections of the central beam are smaller than those of the outermost beams. As described in the U.S. specification No. 4,196,370, this is corrected inter alia by providing around the outermost beams annular elements of a material having a high magnetic permeability (for example, of mu-metal). As a result of these elements, the edge field produced by the field deflection coil is screened somewhat at the area of the outermost electron beams, as a result of which these beams are deflected slightly less and the coma error is reduced.
There also exist deflection coils having convergence errors in which the frame of the central electron beam is too large horizontally and is too small vertically. Correction is obtained, for example, by strip-shaped field shapers as described in U.S. specification No. 4,142,131 which may be deemed to be incorporated herein by reference.
With some deflection coils, removal of the horizontal coma error, causes overcompensation of the vertical coma error, so that vertically a new coma error of opposite sign is formed.